1. Field of the Invention
The present relates to a wireless communication method, access point, wireless communication station and wireless communication system, for performing wireless communication based on a packet transmission system which allocates wireless bands by CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance).
2. Related Background Art
Recently wireless LAN interfaces are installed in various devices. In particular, the popularization of the IEEE 802.11 standard for personal computers, home electronic products and mobile terminals is remarkable. When a mobile terminal is used as a wireless communication station based on the IEEE 802.11 standard, the critical technology deals with power saving. Document “ANSI/IEEE std 802.11, Wireless LAN medium access control (MAC) and physical layer (PHY) specifications, 1999” describes the technology to implement power saving in wireless communication stations, using the following method.
In a case of one access point communicating with one or more subordinate wireless communication stations, if at least one of the wireless communication stations operates in PS (Power Save) mode, a broadcast packet or a multicast packet is transmitted after sending a special beacon called “DTIM” (Delivery Traffic Indication Message). Here “PS mode” refers to a mode in which a wireless communication station performs communication regularly repeating the Awake (wireless interface ON) state and the Doze (wireless interface OFF) state. The wireless communication station notifies the access point whether the wireless communication station is operating in Active mode (wireless interface is constantly ON) or in the above mentioned PS mode. In this notice, the wireless communication station shows that this wireless communication station is operating in PS mode when a power management bit in the frame control field in the MAC header is “1” in the packet to be transmitted to the access point, and makes use of the fact that this wireless communication station is operating in Active mode when the power management bit is “0”. By this, the access point can recognize whether each of the subordinate wireless communication stations is operating in PS mode or in Active mode. If the access point recognizes that one of the subordinate wireless communication stations is operating in PS mode, based on the above method, the access point transmits a broadcast packet or a multicast packet from buffering after transmitting the DTIM beacon.
In the above mentioned prior art, if the wireless communication station to receive multicast service is operating in PS mode, a multicast packet is transmitted after transmitting the DTIM beacon, therefore the multicast packet is transmitted only in the cycle of the DTIM beacon. Generally speaking, the beacon cycle is set to about 100 micro seconds, so the cycle of the DTIM beacon is about several hundred micro seconds. Hence if the multicast service which the radio communication terminal is receiving is such a real-time application as video streaming, quality deterioration as seen in delay jitters may be generated.
This problem occurs because a method for the wireless communication station intermittently receiving multicast packets at an intermittent reception timing, which does not depend on the beacon cycle or DTIM beacon cycle, is not specified in the IEEE 802.11 standard.